In the oil and gas industry, hydrocarbon reservoirs have conventionally been accessed by vertical or near-vertical bores. With finite reserves and a high demand for supplies, extraction of oil and gas from less-accessible reservoirs has become more widespread. To reduce cost, technologies have been developed which allow these reservoirs to be accessed by existing well bores using deviated drilling techniques, that is the reservoir is accessed by drilling a bore at an angle from an existing vertical bore.
Tools that have conventionally been used in vertical or near-vertical bores may encounter problems when used in bores that deviate from vertical. Such tools are lowered in to the bore as part of a tool string and utilises gravity to facilitate transport. In non-vertical bores the gravitational force may be negated by frictional forces resisting movement between the tool string and the walls of the bore. Furthermore, particularly in open hole, the tool string outer surface can stick to the wall of the well bore.
In an attempt to alleviate such problems, the present applicant has proposed a system that reduces friction. Such a downhole device incorporating, for example, a roller is disclosed in the applicant's international application WO 2006/016155. Accordingly, equipment can be more easily transported along bores (cased, open hole or a cased to open hole cement pocket for example) where the increased friction caused by deviation from vertical is minimised by the rolling contact. The use of such equipment helps increase the accessibility of non-vertical bores.
In addition to the increased friction in such bores due to an increased horizontal gradient component, the movement of equipment along such bores may be impeded further by the presence of intermediary obstacles. Wash-outs, sharp bends, misaligned tubular joins, cased to cement pocket entries, uneven surfaces and the like may present sporadic increased resistance to the movement of such equipment. Particularly where equipment first encounters such obstacles, the localised increased friction may impede the movement of equipment through a bore.